Widespread use of molecular beam epitaxy for III-V components has led to the development of new effusion cells for arsenic and phosphorus. Such cells make it possible to obtain the fluxes of arsenic and phosphorus that are required for epitaxial growth of materials such as GaAs, GaAlAs, InP, GaP, GaInP, etc.
Over the last few years, the improvements researched by the manufacturers RIBER, VG, and INTEVAC (formerly VARIAN) have had two objectives:
a) Making high-capacity cells enabling arsenic and phosphorus to be used in charges of about 300 grams (g). This improvement makes it possible for the enclosure to be used between two openings thereof for considerably longer periods of time.
b) Developing effusion cells including high-temperature (900.degree. C.) decomposition heads for decomposing As.sub.4 and P.sub.4 molecules into As.sub.2 and P.sub.2.
A typical known cell includes two regions heated to different temperatures:
a low-temperature (about 300.degree. C.) sublimation chamber in which solid arsenic or phosphorus is sublimed to form tetramers by means of the following reaction: EQU As.sub.sol .fwdarw.As.sub.4 EQU P.sub.sol .fwdarw.P.sub.4 ; and
a decomposition head in which the following decomposition operation is performed at about 900.degree. C.: EQU As.sub.4 .fwdarw.As.sub.2 EQU P.sub.4 .fwdarw.P.sub.2.
Using dimer molecules, in particular P.sub.2, enables the growth conditions of phosphorus-based materials to be considerably improved by increasing the bonding of the phosphorus. This results in a very significant reduction in phosphorus consumption (by a factor of 10).
Cells currently developed by manufacturers incorporate those two important improvements. But it remains difficult to control the flow-rates of As.sub.2 and P.sub.2 to within 1% when growing alloys of the GaInAsP type, or to obtain rapid variations in two distinct flow-rates of As.sub.2 and P.sub.2, such variations being necessary for making certain heterostructures or alloys based on GaInAsP.
Currently, "MOMBE" or "EJMSG" techniques using the gaseous hydrides arsine and phosphine enable the above-described drawbacks to be mitigated. But this requires highly toxic hydrides to be used, and costly precautions must be taken.
An object of the present invention is to provide a simply-made cell making it possible to control the flow-rate of molecular beams rapidly and accurately without using dangerous substances.